Single crystalline wafers comprising semiconducting materials are used as substrates for the manufacturing of micro-electronic components such as field effect or hetero-bipolar transistors, and of opto-electronic components, such as laser or luminescence diodes. By means of distinct processes such as CVD, MOCVD, LPE, MBE the functional layers are deposited and optionally reworked upon those substrates, or are generated within the substrate by means of ion implantation. These substrates then run through complex structuring processes undergoing multiple applications of exposure masks.
For the purpose of orientation (adjustment) of the exposure masks and optionally for the necessary distinction between a front face and a rear face of the substrate, the substrate comprises a so-called orientation flat (OF) and an identification flat (IF), which is offset with regard to the orientation flat by 90° in the clockwise or counter-clockwise direction. The wafer normal and the surface normal of the flats are generally perpendicular with respect to each other. Conventional manufacturing processes of wafers having flats include the generation of the flats by means of grinding. The adjustment accuracy of the orientation flats with regard to the crystallographic <110>-direction may amount to ±1°, for the identification flat ±5°, in case of a conventional wafer manufacturing, but values even up to ±0.02° may be achieved for the orientation flat employing the above method. Flats manufactured by grinding may comprise mis-orientations fluctuating along the flats as well as chips at the edges, which affect the function of the flats as references for the adjustment of exposure masks. This is particularly valid in the case of manufacturing laser diodes, which necessitate high precision and further undisturbed, sharp-edged flats having an adjustment accuracy of ≦|0.02°| over a length relevant for the respective technology.
It is well-known that the orientation accuracy of the flats can be increased, if these are generated by cleaving the generally brittle semiconductor materials instead of grinding the flats, thereby employing natural cleavage planes. For example, in the case of III-V semiconductors, the {110}-planes are natural cleavage planes. From U.S. Pat. No. 5,279,077 and U.S. Pat. No. 5,439,723 there are known wafers provided with flats generated by such cleaving accordingly. From U.S. Pat. No. 5,154,333 there is known a cleaving device, which is used to carry out cleavage with a predefined bending stress of a wafer, which contains a seed crack generated by scribing the wafer. However, a disadvantage of the mechanical cleaving device arises in that the crack progress can not be controlled, and in that due to the initialization of the breakage by means of a seed break at the wafer edge a complex fracture mode is realized.
Alternatively there are thermal dividing methods, which deal with a combination of local heating and neighboring local cooling. A basic method relates to a process of cutting flat glass by means of thermally induced stress which is described in DE 28 13 302. According to this method glass is heated in one area and cooled in another area, both areas being provided on the glass on at least one of its two main faces and being placed one after the other on the intended straight cutting line, and further being sharply delineated and symmetric with respect to the cutting line. Temperature gradients result in the glass along the thickness direction and in the direction of the scribe line. The temperature gradients cause thermal stress, which starting from the initial crack at the edge drives a crack perpendicular to the main faces and along the predetermined straight cutting line. Therein the crack progress velocity can be controlled by regulating the applied temperatures and the feeding of the heating and cooling device.
From WO 93/20015 there is known a method of dividing semiconductor components. According to this method the formation of tensile strength in a region between a heating laser beam and a subsequent cooling initiates a breakage. Using this method, the shape, direction, depth and propagation velocity as well as the accuracy of the breakage generated by thermal stress shall be controlled.
One feature of known methods of dividing brittle materials by propagating a crack is the generation or the presence of an initial crack. In the vicinity of the tip of the crack a stress field is generated using a suitable method such as, e.g., the above mentioned mechanical bending stress, or by applying a thermal load. This stress field leads to a complex load at the propagation front of the crack, which may be characterized by a stress-intensity factor K. If the stress field is selected, such thatK>KC  (1.1)wherein KC is the critical stress-intensity factor specific for the material, the crack length increases until the conditionK≦KC  (1.2)is fulfilled. K>KC is the propagation condition of the separation process, which has to be maintained continually or in intervals, until the complete separation is achieved.
The crack propagation proceeds according to the principle of minimizing the free energy of the body that is to be divided. This means, that the crack propagates such that in isotropic materials the mechanical-energy-release rate G becomes a maximum. In anisotropic materials a principle of minimizing the effective surface energy 2γe of the divided surfaces competes with the principle of maximizing the rate of the mechanical energy release when minimizing the total energy of the system during the crack propagation.dU/dC=2γe−G→Min  (1.3)wherein U denotes the total energy of the system, and C denotes the surface area generated by means of crack propagation.
This means, that with regard to the above-mentioned methods the propagation of the crack is controlled by the time and position dependent stress fields in the case of isotropic materials. If crystallographic cleavage planes are present, which are characterized by a minimum effective surface energy, the direction of crack propagation enforced by the tensional fields (G→max) competes with the directions of these crystallographic cleavage planes upon crack propagation. As a consequence, employing the above-described methods one is restricted in practice to produce only such cleavage planes, which comprise steps or leaps into neighboring lattice planes. This may exert disadvantageous effects on technologically relevant cleavage surfaces such as resonator surfaces of laser components, or on accuracy of the orientation of cleaved flats.
In theory the production of completely planar cleavage surfaces could be achieved where a planar tensile stress field perpendicular to a desired cleavage plane is generated. In this case the stress fields and the cleavage plane would be coordinated with each other to such an extent that these do not compete with each other. However, these conditions can not be kept in practice. For example, it would have to be required in the case of cleaving laser components, that the wafer must be oriented with high precision with respect to the cleavage device, in order to achieve a sufficient quality of the cleavage plane at least over the length of the laser component. Even with the smallest deviations of the wafer orientation with respect to the generated tensional fields, steps formed on the cleavage plane will become inevitable.